1939: Seismology

The year had one major earthquake causing great destruction, that of Central Chile on January 24. The other most noteworthy earthquakes of the year were March 20 and May 1 in Japan; June 18 at Accra, Gold Coast, Africa; and Sept. 22 at Izmir (Smyrna), Turkey.

In the United States there was a destructive earthquake in western Washington on November 13; others occurred on May 4 near Boulder Dam, in northeastern Alabama May 5, and southern New Jersey November 14. On Oct. 18 a shock originating at the confluence of the St. Lawrence and Saguenay Rivers in Canada was felt over a large part of New England.

There are more than one hundred seismograph stations of various kinds in the United States and the regions under its jurisdiction. The number and the principal purpose are about as follows: 39, recording of distant earthquakes; 16, nearby light and moderate earthquake; 51, strong earth motions; and 5, volcanic earthquakes of all types (Hawaii).

The teleseismic (distant earthquake) stations are well distributed; the others, except as mentioned, are chiefly in the Pacific Coast and western mountain regions. The strong-motion instruments are automatic, that is, the earthquake starts the recording which lasts only during the shock. All other instruments are continuously recording and, with few exceptions, photographic recording is used in all types in the United States.

The complete description of an earthquake requires both an instrumental epicenter and information from observers and eyewitnesses. The collection of the latter information from thousands of individuals is well organized. In regions of frequent earthquake, the forms listing the desired information are placed in the hands of individuals in advance; elsewhere they are sent out immediately after the earthquake. Organizations taking part in these varying activities include: the Coast and Geodetic Survey and the Weather Bureau, with collaborating state representatives in the western mountain states; Jesuit Seismological Association; Northeastern Seismological Association; University of California and associated institutions; California Institute of Technology; and the University of Alaska. There are also many scattered activities of universities and scientific institutions.

The purpose of these investigations is to find where and at what depth earthquakes occurred; the extent of area affected and nature of surface effects; better knowledge of the earth's interior and crust, including the relation of earthquakes to faults, or slips at the surface; and the character of the earth motion in the central region of an earthquake.

The last named information together with studies by engineers of earthquake destruction aids in the design of buildings, bridges, dams and other structures to resist earthquake. The periods of buildings and ground vibrations are measured to aid in the solution of the problems. The application of this information which is collected by the Coast and Geodetic Survey and collaborators has been made chiefly at the Massachusetts Institute of Technology, California Institute of Technology, and Stanford University, which also operate shaking platforms of various types on which models of structures are subjected to actual reproductions of the earthquake motion. A number of organizations are working on building codes.

Complete earthquake prediction is impossible, but determination whether stress in the earth's crust is growing probably is not. For this reason crustal movements are measured by geodetic means. Tilt is being measured on the Hayward fault at the University of California. On the same fault near Oakland the noise at the fault is recorded in a deep well, in the expectation that minute slipping preceding an earthquake would cause slight noises. No earthquake has occurred along the fault since observations were started. See also GEOLOGY.